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Eugeo was a kindhearted and considerate person who thought about the needs of others as well as his own. For example, when he first met Kirito during the latter's second dive, Eugeo noticed that Kirito was hungry, and thus shared his food with him. Furthermore, Eugeo always offered to help his valet with her duties and attempted to minimise her workload. He also tried his best to maintain a good relationship with his valet by participating in bonding sessions with her and even scolded Kirito for trying to avoid the sessions.

Eugeo was initially a very compliant person who could not even think of breaking the rules and laws. However, this changed after he witnessed how Raios and Humbert abused their authority as nobles to deceive and abuse his and Kirito's valets to get to him. During this incident, Eugeo managed to break his Seal of the Right Eye, a feat that no Artificial Fluctlight had accomplished before. Eugeo then understood that the law had to be disregarded at times.

Eugeo initially also lacked resolve. Even after his childhood friend was apprehended and taken to the Central Cathedral, he did not have the resolution to follow and save her. He used the excuse of having a Sacred Task so that he never had to think seriously about leaving the village, and thus he spent the following six years trying to cut down the Gigas Cedar. Only after meeting Kirito did Eugeo start taking action. At first, Eugeo greatly relied on his friend to guide him. His resolution was strengthened when he overcame his Seal of the Right Eye and encountered Alice as an Integrity Knight, as he was resolved to even oppose the Axiom Church to save Alice after the incident.

Eugeo was described as a slender-looking[2] boy with soft-looking[2] flax-brown[3], slightly wavy hair[2], green[4] eyes and a cream color skin.[2] Before getting accepted into the Sword Mastery Academy, he wore the same unbleached, rough cotton or flax, short-sleeved,[2] light blue[citation needed] tunic with irregular texture[citation needed], hand-sewn threads on the cuffs[5] and a V-shape cut on the chest, tied with a light brown cord, and trousers as Kirito.[2]

When Eugeo joined the Academy, he wore a standard grey uniform.[6][7] After becoming an Elite Swordsman-in-Training and being given the privilege to customize the color of his uniform, Eugeo chose to wear a blue and gray uniform.[8]

Upon escaping from the prison in the cathedral, Eugeo, along with Kirito, stumbled upon a room filled with weapons and armor, along with a set of clean clothing. Using the opportunity, Eugeo changed his torn and damaged Academy uniform into a blue outfit made from the highest quality materials.[9]

After Eugeo was converted into an Integrity Knight, he temporarily wore an elegant bluish and silvery white armor with a cape.[10]

By the 7th Month of 372 of the Human Empire Calendar, Eugeo had been Kirito's friend since childhood, and had recently received the Sacred Task of cutting the Gigas Cedar tree with him. When Kirito and Alice decided that ice from the Mountain Range at the Edge could help preserve their meals in the hot summer, Eugeo was reluctant to journey to the mountain due to possibly breaking the rules, but was still persuaded to join them.

Eugeo, Kirito and Alice finding the Blue Rose.

They found ice in the cave, along with the bones of a dragon, which was said to have been the guardian of the Human Empire from the forces of darkness, and the Blue Rose Sword, which Bercouli once tried to steal from the dragon. As the sword was too heavy to lift and they did not want to commit grave robbery, they decided to only take the ice that they came for and leave. Unfortunately, after filling their basket with ice, they realize that they cannot remember which cave they used to enter the cave, as the boys had conflicting opinions on the cave they came from.

They chose to try the cave on the nearest side first and Eugeo rushed towards the exit they saw soon after. However, instead of leading back to Rulid Village, the the cave led them to the Dark Territory. At that moment, they witnessed a battle between a Dark Knight and an Integrity Knight. The Dark Knight was being pierced by an arrow from the Integrity Knight's bow and crashed into the ground near the children, who had stopped on the border to the Dark Territory. When Alice attempted to help the dying knight but tripped, Eugeo, who had remained standing without reaction at first, as well as Kirito, tried to reflexively grab Alice by her clothes. However, they only sliced the air with their fingers and thus Alice landed on the ground, touching the soil of the Dark Territory in the process. As entering the Dark Territory was considered a violation of the Taboo Index, Eugeo tried to comfort her by stating that she had only touched the ground, which he believed would not be enough to earn a punishment, but quickly turned to Kirito for support.

The next day, while cutting the Gigas Cedar with Kirito, they noticed an Integrity Knight flying towards the village. After Kirito took the axe off of Eugeo and started running towards the village, Eugeo started chasing after him. When they reached the village and heard that the knight had come to arrest Alice to bring her for interrogation and execution, the two boys attempted to hide Alice but to no avail. Although Kirito encouraged Eugeo to help him save Alice from the Integrity Knight, Eugeo's doubts and a pain in his right eye prevented him from doing anything.

6 years later.

Six years later (in Underworld time), Eugeo was the first person Kirito encountered when he returned to Underworld and helped him find a place to stay in the village. The next day, he showed Kirito the Blue Rose Sword he brought from the cave in the Mountain Range, but they were unable to use it efficiently. He helped Kirito look for Selka when she headed for the cave where Alice touched the ground of the Dark Territory and fought the Goblins in the cave, but was gravely injured. Kirito and Selka saved his life by Selka using a Sacred Art to transfer Life from Kirito to Eugeo. After recovering, he found it easier to wield the Dragon Bone Axe, thus Kirito attempted to use Sword Skills with the Blue Rose Sword again, successfully this time. Eugeo watched this in amazement and asked Kirito to teach him the Aincrad style. With Kirito's assistance he managed to master the use of the Horizontal Sword Skill and cut the Gigas Cedar in the process in just five days. On the festival organized to celebrate his feat, Eugeo revealed his ambition to become a swordsman and wanted to leave the village, but he was challenged by Jink, the town guard, to a duel to decide who would leave the village. With Kirito's advice and his own experience, he easily defeated Jink using Slant, a Sword Skill he learned on his own, and together with Kirito he departed to Zakkaria.

Since leaving the village, Eugeo and Kirito had been working as stable-boys in the «Wolde Farm» on the west outskirts of Zakkaria for about five months, training and waiting for the tournament. On the 28th Day of the 8th Month, the day of the tournament, Kirito and him were about to enter the West gate of the town when a horse went wild and charged at them but they were able to soothe it, and discovered the cause of its distress was a «Large Marsh Horsefly». Kirito gave his suspicions on why one was so far from the nearest swamp and warned Eugeo to be wary of future incidents.

As the pair stopped to get some food, Eugeo got a bit nervous from seeing more people than he was used to in Rulid Village but Kirito tried to get him to relax. After signing up for the tournament, both of them entered the tournament restroom and, having quickly scouted the room, they were relieved that none of the participants were female. While they prepared, Kirito approached Eugeo and warned him of one of the participants, named Egome Zakkalight, whose personality was similar to Jink's.

Both Eugeo and Kirito were able to enter different blocks during the tournament, and were able to pass the preliminaries, and then win the tournament in their respective blocks, thus both became guards. By next spring, each of them received letters of recommendations to attend the Sword Mastery Academy. After they entered the academy, Eugeo became the valet of the 3rd best swordsman in the academy, Gorgolosso Baltoh.

One and a half years after the tournament, Eugeo talked with Kirito during dinner and later helped him with the plants he was growing. While tending the plants, Eugeo asked about Kirito's plans after recovering his memory, to which Kirito replied saying that he would still follow Eugeo. After Kirito requested to borrow Eugeo's sword, Eugeo reminded him of the important event that would take place the next day, the 6th Day of the 3rd Month.

The next day was a rest day, thus the swordsmen went to the city and visited Sadore's shop in the 7th district of North Centoria. Sadore gave Kirito the sword he had crafted from the branch of the Gigas Cedar. After Kirito took the sword, the two trainees returned to the academy.

Later that day, Eugeo overheard some commotion, and discovered that Kirito was going to face Uolo Levanteinn, the head Elite Swordsman-in-Training of the Academy, in a duel. After the duel, Eugeo, along with Gorgolosso, were invited to Sortiliena's room to celebrate the outcome of the duel.

A few months later, Eugeo ranked as an Elite-Swordsman-in-Training in the Academy, and was about to dismiss Tiese, his valet, when he saw Kirito's valet, Ronye, waiting for her mentor to dismiss her. Eugeo offered her to request a change of her mentor, as he saw that Kirito was not a good example. However, Ronye declined the offer and, at that moment, Kirito entered the room from the window with a bag of honey pies for them, and dismissed their valets by telling them to share the pies with their roommates.

After Tiese and Ronye were dismissed, Eugeo reminded Kirito about an advanced Sacred Arts exam that they were going to have the next day and that Kirito had not studied for it yet. While Kirito studied, Eugeo decided to practice in the arena that was exclusive for elite swordsmen.

When Eugeo arrived at the arena, he found Raios and Humbert practicing. After a while of practice, they started mocking Eugeo about his lack of style and mockingly invited him to duel them, but to the nobles' surprise, Eugeo agreed to duel with Humbert, while Raios was left as the referee of the duel. Eugeo managed to make Humbert stumble by connecting two skills and repelling his attack. At this point, Raios stopped the duel before Eugeo was able to land the final strike.

Picnic.

Days later, Eugeo had a picnic with Kirito, Ronye and Tiese where the latter two told their mentors about the difficulties that their friend, Frenica Szeski, was having as Humbert's valet, since he was venting his anger of his duel with Eugeo being a draw on her. Eugeo immediately went with Kirito to talk with Raios and Humbert about the issue, but the two nobles played dumb and provoked Eugeo to mention how improper it was for Humbert to ask his valet to undress to her underwear. This was what the two nobles were waiting for, as Eugeo himself had undressed in front of his mentor many times for his mentor to inspect Eugeo's muscle development. After Raios and Humbert made fun of this, Kirito and Eugeo left with a supposed improvement achieved, as Humbert and Raios left them alone for awhile.

Two days later, the 22nd of the 5th month, was a stormy day and the time for Eugeo and Kirito's weekly sword maintenance in their rooms. As the four-thirty bell rang, they realized that Ronye and Tiese were late for their usual 4 o'clock room cleaning. Eugeo waited in the room while Kirito went out to look for them. But, moments later, Frenica knocked at the door and, to Eugeo's surprise, told him that Ronye and Tiese had gone to Raios and Humbert's room to complain about their actions, because they did not stop harassing her. Having a bad feeling, Eugeo rushed to the room of the nobles, but forgot to leave his sword. With no time to spare, he carried it along.

Eugeo's hesitation.

At Raios and Humbert's room, Eugeo found the two nobles drunk. They took him to one of their rooms where they had Ronye and Tiese bound to the bed, since the two wanted to make him watch as they raped the girls as "punishment" for offending them. Eugeo fought with his conscience and his inability to go against the Taboo Index, while the pain in his right eye increased as he watched the elites' actions. With Tiese's last scream before they were raped, Eugeo finally built up the willpower to go against the rules and attacked the nobles, unintentionally cutting Humbert's arm in the process, while his own right eye was blown away. After this attack, Raios decided to execute him for the crimes of interrupting the punishment and harming Humbert. Eugeo was unable to resist due to his state of shock, although at the last moment, he was saved by Kirito, who blocked the attack. Kirito then dueled with Raios and severed both of his arms, eventually leading to his death.

Eugeo and Kirito were thus branded murderers, and prepped for extraction by an Integrity Knight. However, to Eugeo's shock, the knight that arrived the next morning to retrieve them was none other than his childhood friend Alice, now known as Alice Synthesis Thirty. Eugeo approached her, but was swiftly attacked by Alice and warned not to do so again. Eugeo was disheartened to learn that Alice had lost all memories of her time together with him. Before Alice took Eugeo and Kirito away, Ronye and Tiese arrived just in time to bring them their swords, the Blue Rose Sword and Kirito's black sword, which Alice confiscated.

The two were brought to the Central Cathedral and locked in the dungeon, though they managed to break out thanks to Kirito's quick wit. They escaped to the courtyard, but were confronted by Eldrie Synthesis Thirty-one, with whom they had a heated battle. During the battle, Eugeo recalled that the knight was formerly known as Eldrie Woolsburg, the champion of the latest Four Empires Unity Tournament. As this information caused Eldrie to act strange, Kirito encouraged Eugeo to continue. Moments after Eugeo mentioned Eldrie's mother, they were attacked by another knight. Eugeo and Kirito dashed to avoid the arrows of the knight, until they eventually reached what appeared to be a dead end. But to their surprise, they were called by a girl from a door that they had not noticed before and the two ran into the passageway leading to the Great Library Room.

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While Kirito had a conversation with Cardinal and learnt about the Underworld and Administrator Quinella, Eugeo researched the tomes in the Great Library to learn more about his world's history. Now armed with their weapons' Armament Full Control Art incantations, Eugeo and Kirito left the Great Library and infiltrated the Central Cathedral, determined to reach the top and slay Quinella. Along the way, they defeated the Integrity Knight Deusolbert Synthesis Seven. As Eugeo tries to finish Deusolbert off, Kirito stopped him, telling him that «Aincrad Style» is not for what Eugeo's going to do.

As the duo continued ascending, they encountered Linel Synthesis Twenty-eight and Fizel Synthesis Twenty-nine, who claimed to be sister apprentices. However, just as the two boys discovered that the girls were actually Integrity Knights, they were suddenly paralysed. Soon after the two were dragged by the young Integrity Knights to the fiftieth floor, where Fanatio Synthesis Two and her «Four Oscillation Blades», were waiting, Eugeo witnessed Kirito, who had managed to counter the poison effect, paralyse the two girls from behind with their own weapons. Moments later, Eugeo was given an antidote for the poison by Kirito and ordered to secretly start chanting his Armament Full Control Art as soon as possible. While Kirito battled with the «Four Oscillation Blades» and Fanatio, Eugeo finished his preparations and activated the special ability of his sword when Fanatio recklessly released the full power of her Heaven Piercing Sword. Although the freezing ability of Eugeo's Blue Rose Sword proved to only be a temporary measure against Fanatio, the two managed to defeat her with the special ability of Kirito's sword. After ensuring that Fanatio would not die from her wounds, the duo proceeded to the eightieth floor, the highest floor that the Elevating Disk on the fiftieth floor could take them to.

On the 80th floor, Eugeo and Kirito confronted Alice, but after a few moves against her, quickly realized that they were no match for her. In a desperate attempt, Eugeo unleashed his Armament Full Control Art to restrain Alice while Kirito unleashed his to whittle down her strength, but both Alice's and Kirito's weapons resonated, generating an incredible surge of energy that blew a huge hole in the Cathedral's supposedly invincible walls. The damage to the wall lead to the creation of a gust of wind that carried both Kirito and Alice to the outside as Eugeo watched in horror. Before he could reach them, the wall repaired itself, leaving Eugeo alone in the Cloudtop Garden.

After being separated from Kirito and Alice, Eugeo climbed up the long staircase leading to the 90th floor. There, he found a large white door and entered through it, only to find that the room was a bathhouse. There, he spotted a man, later revealed to be Bercouli Synthesis One, and knowing that he was an enemy, Eugeo engaged him in duel after some preparation. He attempted to counter Bercouli with the Sword Skill «Sonic Leap», but as he rushed forward, Bercouli activated the special ability of the Time Piercing Sword, resulting in Eugeo being seriously wounded. After he managed to stop the bleeding, Eugeo began to activate the Armament Full Control of the Blue Rose Sword. Bercouli noticed this, yet leisurely initiated a chat in order to allow Eugeo to finish chanting the special ability. The battle ensued once more with Eugeo stabbing the Blue Rose Sword into the marble floor, freezing the surroundings with ice and roses as a distraction for Bercouli, and then rushing towards him.

Eugeo's and Bercouli's blades then clashed, which resulted in what appeared to be Eugeo's blade being shattered. However, that was actually an icicle decoy, while the Blue Rose Sword was thrown into the bath before he charged. Eugeo then proceeded to use the Aincrad Skill «Meteor Break» to tackle Bercouli into the water. At that point, Eugeo retrieved his sword and proceeded to stab the bottom of the pool to continue freezing the area around him. Eugeo then activated the second stage of the Blue Rose Sword's Armament Full Control art that trapped both him and Bercouli in thick layers of ice that continuously drained their «Life». When both of them had barely any Life left, Chief Elder Chudelkin appeared and froze Bercouli, all the while mentioning that he had found a very useful pawn whilst looking at Eugeo. Eugeo then lost consciousness, calling out for Kirito and Alice.

Some time later, Eugeo found himself in a room, where he heard his mother's voice. When he asked for the whereabouts of his father and siblings, his mother's voice replied, saying that Eugeo had killed them. At that point, Eugeo noticed that his hands were bloody, and screamed, ending the nightmare. Realizing it was just a dream, he awoke to find Quinella asleep. Despite a voice telling him to get away, Eugeo stood completely still, as Quinella awoke. She began manipulating Eugeo's memories, convincing him that he had never been truly loved, and promising him that he would be loved. Eugeo submitted to Quinella's manipulation, allowing Quinella to access his Fluctlight to convert him into an Integrity Knight.

By the time he met with Kirito and Alice in the 99th floor of the Cathedral, Eugeo had been converted into an Integrity Knight. He no longer held any recollection regarding his two closest friends and merely regarded them as intruders and threats that must be dealt with. He expressed his thanks towards Kirito for bringing the Blue Rose Sword, but as Kirito attempted to hand it over, Eugeo merely made use of an incantation art to snatch it. He then prepared to duel with Kirito as he entered the stance for «Sonic Leap».

While battling with Kirito, Eugeo was able to regain his sense of self, although he could only apologize before freezing Kirito and Alice. He retreated to the top floor and tried to trick his way into assassinating Quinella, but failed. Having broken free from Eugeo's ice, Kirito and Alice reunited with him in Quinella's chambers, where the three of them along with Cardinal confronted Administrator Quinella and Chief Elder Chudelkin. While Alice handled Chudelkin's offensive, Eugeo distracted Chudelkin by firing ice arrows at Quinella, giving Kirito the opening to attack Chudelkin.

After Kirito struck down Chudelkin, Quinella summoned a powerful monster, called a sword golem, which instantly incapacitated Kirito and Alice. Eugeo then heard a voice telling him to stab his dagger into the elevating disk and noticed a small spider on Kirito's right shoulder. After Eugeo stabbed the dagger into the elevating disk, Cardinal used the dagger to create a passageway linking to the Great Library Room that she used to enter the chamber. Although Cardinal healed Kirito and Alice, Cardinal's arrival was part of Quinella's plan to trap her in the room. Finding out that the golem was actually made by transforming three hundred people into swords, she would not be able to kill the golem, thus Cardinal offered her life in exchange for the freedom of the three youths.

As Cardinal's Life was being depleted from the Quinella's attack, Eugeo felt his lack of power and inability to protect the people he cared about. He realized that after their escape, Quinella would turn nearly half of the human population into sword golems to use in the upcoming war. At that moment, Eugeo felt that it was his mission and duty to stop the Administrator and find a way to stop her and save their world. In that instant, he realized that the best way to fight the sword golem was to fuse with his sword.

Eugeo immediately went to the dying Cardinal and convinced her to transform him into a being capable of fighting the sword golem. Cardinal expended the last of her energy to fuse him, the Blue Rose Sword and Alice's extracted memories, in the form of a crystal hidden in the decoration of the chamber's canopy, into a giant sentient sword. In this form, Eugeo easily destroyed the golem, but despite Kirito's warning not to act rashly, Eugeo turned towards Quinella and attempted to kill her. He failed, and the giant sentient sword was split in two, but not before managing to destroy Quinella's weapon and severing her entire right arm. As the sword morphed back into Eugeo, Kirito was horrified at the sight of his friend lying on the floor, his body bisected at the waist. Eugeo poured his dying soul into his broken sword, transforming it into the Red Rose Sword, which Kirito used, along with his black sword, to finally finish off Quinella. Eugeo and a heartbroken Kirito shared one last conversation, and as Kirito finally recalls his memories of Eugeo and Alice during his first dive into Underworld, Eugeo succumbed to his wounds and passed away in Kirito's arms.

As Eugeo's Fluctlight was prepped for deletion from the Underworld's database, he met the personified Alice Zuberg of the extracted memories that were held in Quinella's chambers, determined to follow Eugeo into the afterlife. The two of them express their blessing to Kirito and Alice Synthesis Thirty, and held hands as they passed on.

According to the naming convention in Underworld, each mora (syllable) in a person's name holds the wishes of the parents for their children. The name Eugeo (Yu-ji-o) contains the traits of being gentle, working fast, and honesty.[12]

In an official character popularity poll hosted by Dengeki Bunko for the cover of abec Art Works, Eugeo ranked fifth.[13]